This invention relates to an autoclave furnace especially suitable for use in processes for vacuum sintering of materials followed by hot isostatic pressing. Sintered bodies of near theoretical density may be prepared from particulate matter by sintering under vacuum until open interconnecting porosity connected with the surface has been eliminated and thereafter by hot isostatic pressing until the remaining porosity is removed. The current state of the art is to sinter a partially dense body in a separate vacuum furnace and attain 95% theoretical density; then hot isostatic pressing until achieving 100% theoretical density. The process is old and is taught, for example, in U.S. Pat. No. 3,562,371. However, the process has had a drawback in that separate furnaces were required for the vacuum sintering and the hot isostatic pressing steps. Either a very hot workpiece was transferred from one furnace to another or the workpiece was allowed to cool down prior to transfer. It may be necessary to cool prior to transfer as the sintered workpiece may not be able to stand the thermal shock induced during the hot transfer. Cooling down prior to transfer results in a loss of energy, increases the time required to complete the fabrication process and can change the crystalline characteristics of the sintered body.
This invention relates to a single furnace that may be used for both the vacuum sintering and the hot isostatic pressing steps in the above described process. During the vacuum sinter, the ambient conditions within the workspace of the furnace may be, for example, 1500.degree. C. and a vacuum of 5.times.10.sup.-1 torr. During hot isostatic pressing, the ambient conditions within the workspace may be, for example, 1400.degree. C. and 800 to 1200 bar.
Maintaining uniform temperature in the workspace under vacuum conditions and under pressure conditions requires substantially different approaches. With a vacuum in the furnace, heat can only be transferred by radiation and cannot be transferred by convection. Heat is spread equally in all directions by radiation requiring equal insulation in all directions from the heating elements and workspace. When the furnace is pressurized for isostatic pressures, the heat is mainly transferred by convection which continually tends to move heat upwardly in the workspace. This has both advantages and disadvantages. An advantage, for example, of convection heating is that the bottom of the furnace can be much less heavily insulated and the space just below the bottom of the furnace can be used for a number of functions. For example, the space just below the furnace may be used to contain electrical connections that could not withstand the temperatures within the workspace. However, it is a constant challenge with a pressurized furnace to maintain temperature uniformity in the workspace. These considerations bear upon why the vacuum sintering hot isostatic process has heretofore required separate treating furnaces.
It is an advantage of this invention to provide a single furnace for vacuum sintering and hot isostatic pressing which furnace establishes a uniform heat distribution both when evacuated and when pressurized. The furnace is uniquely structured to enable vapors (outgassing contaminants) that are removed from the workpiece during heat-up and vacuum sintering to be drawn out of the furnace without contacting the heating elements and other functional structure within the furnace that might be damaged thereby. It is yet another advantage of this invention to provide a cold trap within the furnace vessel to condense and collect vapors which might otherwise foul the evacuation system.
It is an advantage of the apparatus claimed herein that the vacuum sintering-hot isostatic pressing process can be practiced less expensively and more expediently. Time and energy is saved by loading and heating but one furnace for both steps. Less capital equipment is required and less auxiliary equipment such as temperature controllers is required.